The present invention relates to a method for the preparation of a frame-supported pellicle for dust-proof protection of a photomask used in the photolithographic patterning work in the manufacturing process of various kinds of electronic devices such as LSIs, VLSIs, liquid-crystal display units and the like. More particularly, the invention relates to a method for the preparation of a frame-supported pellicle which is outstandingly free from the troubles due to dust particle deposition thereon.
As is well known, the so-called photolithographic patterning work is widely undertaken in the manufacture of various kinds of electronic devices such as LSIs, VLSIs, liquid-crystal display units and the like, in which a photoresist layer formed on the surface of a substrate such as semiconductor silicon wafers and base plates of liquid-crystal display units is exposed pattern-wise to actinic rays such as ultraviolet light through a so-called photomask which is a transparency bearing the fine pattern to be formed in the photoresist layer. The pattern on the photomask is so fine that any dust particles deposited on the photomask greatly affect the quality of pattern reproduction resulting in a decrease in the yield of acceptable products.
It is a usual practice accordingly that the photolithographic patterning work is conducted in a clean room freed from dust particles floating in the air as completely as possible but perfect cleanness of photomasks can hardly be obtained even in a clean room of the highest degree of cleanness. Therefore, a conventional means to ensure cleanness of a photomask is to protect the photomask by mounting a frame-supported pellicle thereon, which consists of a rigid frame to be mounted on and fixed to the photomask by means of a pressure-sensitive adhesive and a highly transparent pellicle membrane of a plastic resin adhesively bonded to the frame in a slack-free fashion so that dust particles are never deposited on the surface of the photomask but are deposited on the pellicle membrane. The dust particles deposited on the pellicle membrane have little adverse influences on the quality of pattern reproduction since the light for pattern-wise exposure is focused on the photomask and not on the pellicle membrane at a distance from the photomask.
Needless to say, a primary requirement for the pellicle membranes is that transmission of light for the pattern-wise exposure therethrough is as high as possible. In this regard, several polymeric materials such as nitrocellulose, cellulose acetate and the like are widely used as a material of pellicle membranes. In connection with the method for adhesive bonding of the membrane to the frame, Japanese Patent Kokai No. 58-219023 proposes that a pellicle membrane made from these polymeric materials is attached and bonded to the surface of a pellicle frame, which is made from aluminum, stainless steel, polyethylene and the like, wet with an organic solvent capable of dissolving the polymer of the membrane. Alternatively, the pellicle membrane can be adhesively bonded to the surface of a pellicle frame by using an acrylic or epoxy resin-based adhesive (see, for example, U.S. Pat. No. 4,861,402 and Japanese Pat. Publication No. 63-27707). Besides the above mentioned cellulose derivatives as a material of the pellicle membrane, a proposal has been recently made for the use of an amorphous fluorocarbon polymer (see, for example, Japanese Pat. Kokai No. 3-67262).
The procedure usually undertaken for the preparation of these frame-supported pellicles is as follows. Thus, the above mentioned polymeric resin as the material of the pellicle membrane is dissolved in a suitable organic solvent in a concentration of 3 to 10% by weight and this solution is uniformly spread on a horizontally laid, flat surface of a substrate plate such as a semiconductor silicon wafer, glass plate and the like by using a suitable coating machine such as spin coater, knife coater and the like followed by evaporation of the solvent from the coating layer of the solution on the substrate surface to form a thin dried film of the polymeric resin to serve as the pellicle membrane. Thereafter, a rigid frame made from aluminum, stainless steel and the like is bonded to the resin film still on the substrate surface by using an adhesive followed by separation and removal of the substrate plate from the resin film so as to complete a frame-supported pellicle.
The above mentioned step of separation of the substrate plate from the resin film adhesively bonded to the pellicle frame can of course be performed in an ordinary atmosphere of a clean room provided that the resin film is not firmly bonded by virtue of an appropriate surface-release treatment of the substrate surface. An improvement relative to this separating work of the resin film and the substrate plate has been recently proposed in Japanese Patent Kokai No. 3-67262, according to which the separating work is conducted in water. This method is not free from a problem of dust particle deposition because, in addition to the particles floating in the water bath, any particles formed from the holders of the substrate plate and the pellicle frame in contact with water are released into the water bath and the floating dust particles in water are eventually deposited on the pellicle membrane as well as on the pellicle frame to cause troubles after drying of the pellicle freed from the substrate plate.